In recent years, there have been growing demands for a large capacity and speeding up of wireless communications, and research on methods of improving the effective utilization factor of finite frequency resources have flourished. As one of the methods, attention is focused on a technique of using a spatial domain. An adaptive array antenna (adaptive antenna) is known as one of spatial domain use technologies. With the adaptive array antenna, a weighted coefficient by which a reception signal is multiplied is used to adjust the amplitude and the phase, whereby a signal coming from any desired direction is strongly received and the interference wave direction is suppressed. Accordingly, identical inter channel interference can be decreased and the communication capacity of a wireless communication system can be improved.
Known as other spatial domain use technologies are space division multiple access (SDMA) of transmitting different data series to different terminals and space division multiplexing (SDM) of transmitting different data series to the same terminal using physical channel at the identical time, at the same frequency, and of the same code using spatial orthogonality in a propagation line. If the spatial correlation coefficient between terminals is smaller than a predetermined value, the SDMA technology can be used for improving the throughput and the number of simultaneously accommodated users of a wireless communication system (refer to non-patent document 1).
On the other hand, in the SDM technology, a transmitter and a receiver are provided each with a plurality of antenna elements and SDM transmission is realized in a propagation environment wherein the reception signal correlation between antennas is low (refer to non-patent document 2). In this case, the transmitter transmits different data series using physical channel at the identical time, at the same frequency, and of the same code for each antenna element from the attached antennas. The receiver separates and receives the different data series from the reception signal through the attached antennas. Thus, a plurality of space division multiplex channels are used, whereby speeding up can be accomplished without using a multilevel modulation. To execute SDM transmission, in an environment wherein a large number of scatterers exist between the transmitter and the receiver under a sufficient S/N (signal-to-noise ratio) condition, if the number of antennas of the transmitter and that of the receiver are the same, the channel capacity can be expanded in proportion to the number of the antennas.
Multiuser MIMO (Multiple Input Multiple Output) technology is known as a technology provided by fusing the SDMA technology and the SDM technology (refer to non-patent document 3). The multiuser MIMO technology makes possible space division multiplexing transmission and space division multiple access based on directivity under a condition that the channel matrix of a terminal connected at the same time by way of space division multiplexing is already known in the transmitter. The channel matrix is represented as a channel vector in a terminal including only a single antenna, but is handled as a general channel matrix.    Non-patent document 1: T. Ohgane et al, “A study on a channel allocation scheme with an adaptive array in SDMA,” IEEE 47th VTC, vol. 2, 1997, p. 725-729    Non-patent document 2: G. J. Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Labs Tech. J, Autumn 1996, p. 41-59    Non-patent document 3: Q. Spencer et al, “Zero-Forcing Methods for Downlink Spatial Multiplexing in Multiuser MIMO channels,” IEEE Trans SP, Vol. 52, No. 2, 2004, p. 461-471